Data rate control for event-based vision sensor

ABSTRACT

In dynamic vision sensor (DVS) or change detection sensors, the chip or sensor is configured to control or modulate the event rate. For example, this control can be used to keep the event rate close to a desired rate or within desired bounds. Adapting the configuration of the sensor to the scene by changing the ON-event and/or the OFF-event thresholds, allows having necessary amount of data, but not much more than necessary, such that the overall system gets as much information about its state as possible.

RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/858,840, filed on Dec. 29, 2017, which claims priority to SwissProvisional Patent Application No. CH20160001764, filed on 30 Dec. 2016,and Swiss Provisional Patent Application No. CH20160001765, filed on 30Dec. 2016, all of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

Today, machine vision is mostly based on conventional cameras and theirassociated frame-based image sensors. For some machine vision tasks,e.g., object recognition, these conventional frame-based cameras arewell suited. However, for other tasks, e.g., tracking or position andmotion estimation, the conventional image sensors have drawbacks.

The main drawback is that conventional cameras produce a significantamount of redundant and unnecessary data, which has to be captured,communicated and processed. This high data load slows down the reactiontime by decreasing temporal resolution, results in increased powerconsumption, and increases the size and cost of machine vision systems.In addition, most image sensors suffer from limited dynamic range, poorlow-light performance and motion blur.

These drawbacks arise from the fact that the data is captured as asequence of still images (frames). In some cases, encoding dynamicscenes as still images is useful to produce beautiful images and moviesbut not optimal for data processing, but this is less important for manymachine-vision uses.

Conventional computer vision systems using conventional camerastypically compare features between sequential image frames for objectrecognition. To estimate the position and orientation of a mobile systemand to infer a three dimensional map of the surrounding world, twosequential images, which are partially overlapping but taken atdifferent times and from different poses, are compared. To infer themotion that occurred between the two frames, characteristic visuallandmarks (key points or other visual features) have to be matchedacross the two images. Finding these pairs of points that correspond toeach other in both images is known as solving the “correspondenceproblem.”

Solving the correspondence problem requires significant amount ofprocessing power. To detect landmarks, every pixel in an image may haveto be searched for characteristic features (corners, blobs, edges,etc.). The pixels and their surrounding neighborhood of pixels are thengrouped to characterize the so called feature descriptors, which arethen used for matching the features between the frames and therebyestablishing pairs of corresponding points. This is computationallyintensive. Direct approaches that directly compare pixel intensities arecomputationally even more complex.

On the other hand, the so-called Dynamic Vision Sensor (DVS) or eventbased change detection sensor is a sensor that overcomes the limitationsof frame-based encoding. By using in-pixel data compression, the dataredundancy is removed and high temporal resolution, low latency, lowpower consumption, high dynamic range with little motion blur isachieved. DVS is thus well suited especially for solar or batterypowered compressive sensing or for mobile machine vision applicationswhere the position of the system has to be estimated and whereprocessing power is limited due to limited battery capacity.

The DVS pre-processes visual information locally. Instead of generatingcrisp images, the DVS produces smart data for computer applications.While conventional image sensors capture a movie as a series of stillimages, the DVS detects and only transmits the position of changes in ascene. It thus encodes the visual information much more efficiently thanconventional cameras because it encodes in-pixel data compression.Specifically, rather than encoding frames as image data, the DVS detectschanges and encodes those changes as change events. This means that theprocessing of the data is possible using less resources, lower net powerand with faster system reaction time. The high temporal resolutionallows continuously tracking visual features and thereby overcoming thecorrespondence problem. Additionally, the architecture of DVS allows forhigh dynamic range and good low-light performance.

SUMMARY OF THE INVENTION

A general property of DVS or change detection sensors is that the outputdata rate is variable, because the event rate depends on the scene thatthe sensor is observing, as well as on the sensor configuration.

The variable amount of change events to be read out from the sensor canpose problems on the sensor and at the system level, especially if thereare more events to be read out than what the processor can process inreal-time.

Discarding the events if they cannot be processed leads to data loss andthus the system loses information about its status (e.g. about itsmovement and position). In the case of a change detection sensor, theresult could be that the sensor will miss events occurring in a portionof its pixel field, for example.

Adapting the configuration of the sensor to the scene allows having asmuch data as necessary but not much more, such that the overall systemgets as much information about its state as possible. On the other hand,the number of events can be controlled based on the state of the sensor.For example, to reduce power, it might be desirable to reduce the numberof events such as during a sleep cycle and then increase the number ofevents during normal operating mode.

The system proposed here changes the chip or sensor configuration tocontrol or modulate the event rate. For example, this control can beused to keep the event rate close to a desired rate or within desiredbounds.

Nevertheless, the amount of events generated during a given time slicecan only be determined definitively after outputting and counting them.Here, some embodiments use an event count estimator that allows a roughestimate of the number of events pending before reading them out.

With even a rough estimate of the number of events pending, the systemcan decide if it is able to cope with the data and read them out, or ifthere are too many (or too few) events, the system can redo the changedetection with higher thresholds and so get fewer events (or redo thecomparison with lower thresholds and get more events).

In general, according to one aspect, the invention features a changedetection sensor. The sensor comprises a pixel array comprising pixelsthat detect light and event detectors for detecting events associatedwith light received by the pixels. According to the invention, an eventrate detector is provided for assessing the events. A controller thenchanges how the pixels detect the events based on the event ratedetector.

In one embodiment, the event rate detector comprises a counter forassessing the events by counting the events from the event detectors.

In another embodiment, the event rate detector comprises an eventestimator for assessing the events by estimating events. This can beimplemented by analyzing an analog signal that is based on the number ofevent detectors registering events.

Preferably, the sensor includes a threshold generation circuit forsetting thresholds applied by the event detectors. The controller thenchanges the thresholds provided in response to the assessment providedby the event rate detector.

Preferably, the controller sets thresholds for ON events and OFF eventsseparately based on ON events and OFF events in the pixel array.

In a current embodiment, each of the pixels comprises a memory capacitorof one of the event detectors for storing a charge corresponding toreceived light when the pixel was reset. Each of the pixels may furthercomprise one or more comparators of the event detectors for comparingthe changes in received light to one or more thresholds to detect theevents.

In other embodiments, the comparators are located in a readout circuit.

In general, according to another aspect, the invention features a methodof operation of a change detection sensor. The method comprisesdetecting events associated with light received by pixels in a pixelarray, assessing the events from the pixel array, and changing how thepixels detect the events based on the assessment of the events from thepixel array.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1A is a block diagram of an event based change detection sensorincluding a event rate control.

FIG. 1B is a circuit diagram showing one example of a change detectionpixel that could be used in the pixel array 110.

FIG. 2 is a block diagram showing one embodiment of the event ratedetector 200 implemented as an event counter that is reset periodically.

FIGS. 3A and 3B are flow diagrams illustrating the operation of thecontroller 120 for controlling the threshold generation circuit based onthe event rate detector.

FIG. 4 is a schematic diagram showing an event based change detectionsensor that generates an event rate estimate according to anotherembodiment.

FIG. 5 is a schematic diagram showing an event based change detectionsensor that generates an event rate estimate according to still anotherembodiment.

FIG. 6 is a circuit diagram showing a possible implementation of thecurrent ADC 212 of the previous embodiments.

FIG. 7 is a state diagram showing a possible implementation of thecontroller 120.

FIG. 8 is a block diagram showing a basic threshold generation circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A is a block diagram of an event based change detection sensor 100having event rate control.

In general, the change detection sensor 100 include five elements thatare specifically relevant to the present system:

array of change detection pixels 110;

an event rate detector 200;

a controller 120;

a threshold generation circuit 130; and

a readout circuit 140.

The pixel array 110 generates a variable amount of data: change events.The event rate detector 200 periodically assesses the events such as bycounting or estimating the change events. Then the controller 120 adaptsthe sensor configuration so that in the next time step the pixel array110 generates an amount of change events closer to the desired rate.

The pixel array can be any array of change detection pixels such asdescribed in U.S. Pat. Appl. No. US 2008/00135731, by Lichtsteiner, etal., entitled “Photoarray for Detecting Time-Dependent Image Data”,which is incorporated herein by this reference; or such as described inU.S. patent application Ser. No. 15/858,427, entitled Dynamic VisionSensor Architecture, filed on Dec. 29, 2017, (hereinafter the BernerPatent Document, which document is incorporated herein by this referencein its entirety); or such as those disclosed in J. Kramer, “An on/offtransient imager with event-driven, asynchronous read-out,” IEEEInternational Symposium on Circuits and Systems, 2002, vol. 2, pp.165-168, 2002) or alternatively spatial contrast detection pixels suchas in Ruedit et al., “A 128×128 pixel 120-dB dynamic-range vision-sensorchip for image contrast and orientation extraction,” IEEE Journal ofSolid-State Circuits, vol 38, no 12, pp 2325-2333, December 2003.

FIG. 1B is a circuit diagram of one possible change detection pixel 112.

This exemplary pixel 112 of the two dimensional array 110 includes aphotosensor such as a photodiode PD that detects or measures impinginglight and converts the light intensity into a signal. Here the signal isa current, Iphoto. The pixel 112 also has an event detector 114 thatthen determines whether there has been a sufficient change in the lightdetected by the photosensor such that an event has occurred. There isthis accomplished by monitoring the current Iphoto.

In more detail, the exemplary event detector 114 includes aphotoreceptor circuit PRC that then generates a photoreceptor signal Vprdependent on the light intensity. A memory capacitor C1 then rememberspast photoreceptor signals.

The comparator A1 compares the difference between current photoreceptorsignal Vpr and past photoreceptor signal to a threshold Vb. Thisthreshold Vb is supplied by the threshold generation circuit 130.

Preferably, the comparator A1, or a pair of comparators, detect ONevents, based on an ON event threshold, and OFF events based on an OFFevent threshold. ON events are characteristic of an increase in thelight received by the photosensor, and OFF events are characteristic ofa decrease in the light received by the photosensor. If a singlecomparator is used, then the ON event threshold and the OFF eventthreshold are provided serially in time as threshold Vb.

This comparator A1 can be in each pixel, or shared between a subset (forexample a column) of pixels. In one case, the comparators are shared andlocated in the readout circuit 140. Different examples of such aconfiguration are disclosed in the Berner Patent Document.

In the preferred embodiment, however, the comparator A1 will be integralto the pixel 112, with each pixel having one or more dedicatedcomparators A1.

Memory 50 stores the event or events. The events are taken from thecomparator output Vcomp, based on a sample signal from the controller120. Memory can be a sampling circuit (for example a switch and aparasitic or explicit capacitor) or a digital memory circuit (a latch ora flip-flop). In one embodiment the memory will be a sampling circuitand each pixel will have two memories, one for storing any ON event, andone for storing any OFF event. The memory supplies the output on one ormore lines Evt.

A conditional reset circuit R1 provides a conditional reset based on acombination of the state of the memorized comparator output and a resetsignal GlobalReset applied by the controller 120.

The pixel circuit 112 and controller 120 operate as follows.

A change in light intensity received by the photosensor PD willtranslate to a change in photoreceptor signal Vpr. When the resetcircuit R1 is not conducting, the changes in Vpr will be reflected alsoin the voltage Vdiff at the inverting input (−) to the comparator A1.This occurs because the voltage across the memory capacitor C1 staysconstant.

At times selected by the controller 120, the comparator A1 of the eventdetector 114 compares the voltage at the second terminal of the memorycapacitor C1 (Vdiff) to a threshold voltage Vb (from the thresholdgeneration circuit 130) applied to the non-inverting input (+) of thecomparator A1.

The controller 120 operates the memory 50 to store the comparator outputVcomp. The memory 50 is typically implemented as part of the pixelcircuit 112 as shown.

In other embodiments, however, the memory 50 is implemented as part ofreadout circuit 140 (peripheral circuit, one per each column of thepixel array 110).

In the illustrated embodiment, the sensor's readout circuit 140 readsout the memories 50 for each of the pixels 112 in the array 110. In oneexample, the readout circuit 140 stores the coordinates of the pixels inthe array that have detected change events.

If the state of the stored comparator output held in the memory 50indicates sufficient change in light intensity (i.e., an event) AND theglobal reset signal GlobalReset signal from the controller 120 isactive, the conditional reset circuit R1 is conducting. Here “AND”indicates the logical AND operator. With the conditional reset circuitR1 in a conductive state, the voltage at the inverting input of thecomparator A1 (Vdiff) is reset to a known level. Thus, it stores thecurrent photoreceptor signal Vpr on the memory capacitor C1.

Event Counting

FIG. 2 shows one embodiment of the event rate detector 200.

Here, the event rate detector 200 is implemented as a counter 210 thatis periodically reset by the controller 120. This counter is incrementedbased on the events read out by the readout circuit 140. The event countis provided to the controller 120. The counter 210 is in turn reset bythe controller 120 when a new count is required.

In one example, the event rate detector 200 counts the number of eventsoutput by the sensor array 110 during a given time window. Here, this isaccomplished such that the readout circuit sends an Increment signal tothe counter 210 every time it reads out an event from the pixel array110.

In one specific example, the event rate detector 200 counts the numberof both ON events output by the sensor array 110 and OFF events outputby the sensor array 110, during a given time window. This is preferablyaccomplished using a separate ON event counter and OFF event counterwithin counter 210.

FIG. 3A is a flow diagram illustrating the operation of the controller120 for controlling the threshold generation circuit 130 based on theevent rate detector 200 according to one embodiment.

Specifically, in step 310, the counter 210 of the event rate detector200 is reset by the controller 120 when a new count is required.

In step 312, the controller 120 reads out the count of the events aftera predefined time.

Then, in step 314 the controller calculates a change in the eventthreshold that is to be applied to the pixel array 110. This newthreshold is sent to the threshold generation circuit 130 in step 316.Then the threshold is applied to the event detectors 114 of the sensorarray 110 as threshold Vb in determining the next set of events.

FIG. 3B is a flow diagram illustrating the operation of the controller120 for controlling the event rate detector 200 according to anotherembodiment.

In this embodiment, the controller 120 waits for an external trigger instep 318 before resetting the counter in step 310. This embodiment wouldbe most useful in the situation in which the sensor should besynchronized to an external timing source.

Event Count Estimator

A second approach relies on generating an event rate estimate. Thisapproach employs an event count estimator within the event rate detector200. It avoids having to read all the events from the array to determinethe number of events. Instead, this approach allows roughly estimatingthe number of events, before the events are read out or as part of theread out process.

FIG. 4 is a schematic diagram showing an event based change detectionsensor 100 that generates an event rate estimate according to anotherembodiment.

In more detail, an event detector 114 is located in each pixel 112 ofthe array 110 and is part of each pixel circuit, along with aphotoreceptor 116 for detecting incident light. A current source 118 ofeach pixel circuit 112 is enabled only if an event is present in thispixel.

Specifically, if the event detector 114 registers a change in the lightreceived by the photorsensor 116, such as due to an ON event and/or anOFF event, then the event signal Evt becomes active, which closes theswitch 117 associated with that pixel 112.

The current of all pixels 112 of the array 110 or a sampling of pixelsdistributed through the extent of the array 110 of the sensor 100 issummed up and fed to a current analog to digital converter (ADC) 212 ofthe event rate detector 200. This ADC 212 takes the current and convertsthe magnitude of the current into a digital representation. This digitalrepresentation of the sum of the currents is then provided to thecontroller 120 as an estimate of the number of detected events, and ifperformed over a known period of time, it is indicative of the eventrate.

In general, the current sources 118 will not be very well matchedbetween pixels 112. This means that the sum of the currents will notallow an exact estimation of how many pixels have an event pending, butit will allow a rough estimate.

FIG. 5 is a schematic diagram showing a change detection sensor thatgenerates an event rate estimate according to another embodiment.

In more detail, a photorsensor 116 of each pixel 112 generates a signalthat is responsive to and indicative of the instantaneous amount oflight detected by that photoreceptor 116. This information is sent tothe event detector 114.

Thus, the output of the comparator in the event detector 114 changeswhen the light received by the photoreceptor 116 has changed by anamount greater than the threshold with respect to the last time it wasreset. Often, an ON event threshold is supplied to the pixel as thePixel Threshold to discriminate ON events, which are associated with anincrease in the amount of light detected by the photoreceptor 116. Thenan OFF event threshold is supplied to the pixel as the Pixel Thresholdto discriminate OFF events, which are associated with a decrease in theamount of light detected by the photoreceptor.

In other cases, the event detector includes two comparators, a firstcomparator that receives the ON event threshold to discriminate ONevents, and a second comparator that receives the OFF event threshold todiscriminate OFF events.

If the signal Enable evt counter is active (and thus Disable evt counterinactive), all the readout lines are shorted together by the closing ofswitches 188 and the readout lines are disconnected from the columnreadout circuits 180 by the opening of switches 190. With all thereadout lines connected to the current ADC 212, a current will flow fromthe current ADC 212 to all pixels in which the comparator output is highand Evt is active to thereby close the switches 117 in each pixel 112.Further, the OR gates 186 for each row will close the switches 178 ineach pixel 112 since Enable Evt counter is active. So the total currentwill depend on the number of pixels in which the output of thecomparator 174 is high or signal line(s) Evt is/are active.

If the signal Enable evt counter is inactive (and thus Disable evtcounter active), the pixel readout lines are connected to theircorresponding column readout circuits 180 if the corresponding Readoutrow select X is active.

FIG. 6 shows a possible implementation of the current ADC 212 of theprevious embodiments.

The summed current is fed to an operational amplifier A 214 with aresistor R 216 in a negative feedback configuration. The voltagedifference between the reference voltage and the voltage at the outputof the operational amplifier is proportional to the magnitude of theinput current and represents thus the number of pending events.

The voltage at the output of the operational amplifier is then fed to aconventional voltage ADC 218, which can be implemented as a flash ADC, aSAR ADC, a sigma-delta ADC, etc.

After the current to voltage conversion, a voltage sampling step(between the out of the OpAmp 214 and the voltage ADC 218) can be added,so that the ADC conversion, which can be long, can be done while thepixels are read out. Once the ADC conversion is finished, the readout ofthe pixels can be stopped if needed.

Controller 120

The controller 120 is most likely a digital block that has as input theevent rate detector 200 and a desired event rate (or bounds for thedesired event rate) from outside the sensor 100, and optionally atrigger input. The controller 120 outputs control signals for the chipconfiguration (i.e., time resolution and threshold to the thresholdgeneration circuit 130).

The controller 120 can be implemented as a traditional P, PI or PIDcontroller, or more simply as a state machine.

However, when using an event rate estimator, the controller 120 couldalso be implemented as an analog circuit. The input to this analogcontroller then would not be a digital representation; instead theoutput voltage of the operation amplifier 214 in FIG. 6 would typicallybe directly fed to the controller 120.

FIG. 7 is a state diagram for the controller 120.

The event threshold is modified to hold the event rate within desiredbounds based on the signaling from the event rate estimator.

-   -   Event rate: input from the event rate detector 200.    -   MinEventRate: constant, minimal desired event rate.    -   MaxEventRate: constant, max desired event rate.    -   MinEventThreshold: parameter, minimal threshold setting.    -   MaxEventThreshold: parameter, max threshold setting.    -   Step: parameter, step by which the event threshold is changed        during each pass of the loop.    -   EventThreshold: output fed to event generation circuit.    -   Trigger: periodic input signal that starts the loop.

In more detail, the controller 120 waits for the trigger in state 710.When it is received, the controller 120 compares the event rate providedby the event rate detector 200 to a minimum event rate in state 712. Ifthe event rate is smaller than the minimum event rate then thecontroller proceeds to state 716. In state 716, the controller 120compares the event threshold to a minimum event threshold; if it issmaller, then it returns to the wait state in 710. On the other hand, ifthe event threshold is larger than the minimum event threshold then theevent threshold is reduced in state 718 and it returns to the wait state710.

On the other hand, if in state 712, the event rate is larger than theminimum event rate then the event rate is compared to the maximum eventrate in state 714, if it is smaller, then the controller 120 returns tothe wait state 710. On the other hand, if the event rate is larger thanthe maximum event rate then the event threshold is compared to themaximum event threshold in state 720. If the event threshold is largerthan the maximum event threshold in state 720, then the controller 120returns to the wait state 710. On the other hand, if the event thresholdis smaller than the maximum event threshold then the event threshold isincreased by a step in state 722 and returns to the wait state 710.

Threshold Generation Circuit

For change detection pixels such as those disclosed in U.S. Pat. ApplNo. US 2008/00135731 by Lichtsteiner, et al., entitled “Photoarray forDetecting Time-Dependent Image Data”, and incorporated herein by thisreference, the event thresholds are the difference between a thresholdvoltage and an amplifier bias voltage. The On Threshold voltage isusually higher than the bias voltage, while the Off Threshold voltage islower.

Basic Threshold Generation

FIG. 8 shows a basic threshold generation circuit which is a set ofvoltage digital to analog converters (DACs) or current DACs 810, 812,such as those disclosed in P. Lichtsteiner, C. Posch, and T. Delbruck,“A 128×128 120 dB 15 μs Latency Asynchronous Temporal Contrast VisionSensor,” IEEE J. Solid-State Circuits, vol. 43, no. 2, pp. 566-576,February 2008; and T. Delbruck, R. Berner, P. Lichtsteiner, and C.Dualibe, “32-bit Configurable bias current generator withsub-off-current capability,” in Proceedings of 2010 IEEE InternationalSymposium on Circuits and Systems, 2010, pp. 1647-1650. Specifically,for the set of DAC's 810, 812, 814, there is one for generating thecomparator bias voltage 814, one for the OnThres voltage 810 and one forthe OffThres voltage 812.

Event Rate and Balance Control

In a further embodiment, the sensor 100 counts ON (increasing lightintensity) and OFF (decreasing light intensity) events, separately. Thenthe ratio between ON and OFF events is used for controlling the ON andOFF thresholds separately. In general, it is desired a similar amount ofON events and OFF events over a large (larger than seconds) time window.

No-Motion Threshold

A property of event-based change detection sensors is that they do notgenerate data (or very little data) if there is no motion in the scene.In such a case of no motion, it may not be desirable to change thethreshold, so the controller may be extended to include a minimal eventrate threshold, below which the control loop to change theEventThreshold output is not activated.

Time Resolution Control

In some pixel designs, the time bin rate (rate of comparison) can bechanged. In a pixel such as proposed by described in U.S. Pat. Appl No.US 2008/00135731, the refractory bias can be controlled, which allowslimiting the rate at which a pixel can produce events.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A change detection sensor, comprising: a pixelarray comprising pixels that detect light, wherein the pixels comprisephotosensors and photoreceptor circuits that output first signals basedon the response of the photosensors; event detectors for detectingevents associated with light received by the pixels by comparing thefirst signals and a first reference voltage; an event rate detector forcounting the events with a counter; and a controller for changing howthe pixels detect the events based on the event rate detector bychanging the first reference voltage; wherein the controller compares anevent rate provided by the event rate detector to a maximum event rate,if the event rate is larger than the maximum event rate the controllerchanges a time resolution of the sensor.
 2. The sensor as claimed inclaim 1, further comprising a threshold generation circuit for settingthresholds applied by the event detectors by providing the firstreference voltage.
 3. The sensor as claimed in claim 1, wherein thecontroller changes the first reference voltage in response to theassessment provided by the event rate detector.
 4. The sensor as claimedin claim 1, wherein the controller sets thresholds for ON events basedon ON events in the pixel array based on the counter.
 5. The sensor asclaimed in claim 1, wherein each of the pixels comprises a memorycapacitor of one of the event detectors for storing a chargecorresponding to received light when the pixel was reset.
 6. The sensoras claimed in claim 1, wherein each of the pixels comprises one or morecomparators of one of the event detectors for comparing the changes inreceived light to the first reference voltage and a second referencevoltage to detect the events.
 7. The sensor as claimed in claim 1,further comprising comparators of the event detectors for comparingchanges in received light to the first reference voltage to detect theevents, the comparators being located in a readout circuit.
 8. A methodof operation of a change detection sensor, the method comprising:detecting light with pixels of a pixel array, wherein the pixelscomprise circuits that output first signals based on the response ofphotosensors of the pixels; detecting events associated with lightreceived by the pixels by comparing the first signals and a firstreference voltage; counting the events with a counter with an event ratedetector; changing how the pixels detect the events based on the eventrate detector by changing the first reference voltage; comparing anevent rate provided by the event rate detector to a maximum event rate;and if the event rate is larger than the maximum event rate, changing atime resolution of the sensor.
 9. The method as claimed in claim 8,further comprising changing the first reference voltage in response tothe assessment provided by the event rate detector.
 10. The method asclaimed in claim 8, further comprising setting thresholds for ON eventsbased on ON events in the pixel array based.
 11. The method as claimedin claim 8, wherein each of the pixels comprises a memory capacitor forstoring a charge corresponding to received light when the pixel wasreset.
 12. The method as claimed in claim 8, wherein each of the pixelscomprises one or more comparators for comparing the changes in receivedlight to the first reference voltage and a second reference voltage todetect the events.
 13. The method as claimed in claim 8, furthercomprising comparing changes in received light to the first referencevoltage to detect the events using comparators located in a readoutcircuit.
 14. A change detection sensor, comprising: a pixel arraycomprising pixels that detect light; event detectors for detecting ONevents associated with light received by the pixels; an event ratedetector for assessing the events; and a controller for changing how thepixels detect the events based on the event rate detector.
 15. Thesensor as claimed in claim 14, wherein the event rate detector comprisesa counter for assessing the events by counting the events from the eventdetectors.
 16. The sensor as claimed in claim 14, wherein the event ratedetector comprises an event estimator for assessing the events byestimating events.
 17. The sensor as claimed in claim 14, wherein theevent rate detector assesses the events by analyzing an analog signalthat is based on the number of event detectors registering events. 18.The sensor as claimed in claim 14, further comprising a thresholdgeneration circuit for setting thresholds applied by the eventdetectors.
 19. The sensor as claimed in claim 14, wherein the controllerchanges the thresholds provided in response to the assessment providedby the event rate detector.
 20. The sensor as claimed in claim 14,wherein the event detectors detect both the ON events and OFF events inthe pixel array.
 21. The sensor as claimed in claim 14, wherein each ofthe pixels comprises a memory capacitor of one of the event detectorsfor storing a charge corresponding to received light when the pixel wasreset.
 22. The sensor as claimed in claim 14, wherein each of the pixelscomprises one or more comparators of one of the event detectors forcomparing the changes in received light to one or more thresholds todetect the events.
 23. The sensor as claimed in claim 14, furthercomprising comparators of the event detectors for comparing the changesin received light to one or more thresholds to detect the events, thecomparators being located in a readout circuit.
 24. The sensor asclaimed in claim 14, wherein the controller sets thresholds for the ONevents and OFF events separately based on ON events and OFF events inthe pixel array.
 25. The sensor as claimed in claim 14, wherein theevent rate detector comprises a counter for assessing the events bycounting the events from the event detectors and the counter isperiodically reset by the controller.
 26. The sensor as claimed in claim14, wherein the event rate detector counts a number of both the ONevents output by the pixel array and OFF events output by the pixelsensor array, during a given time window using a separate ON eventcounter and OFF event counter within the counter.
 27. The sensor asclaimed in claim 14, wherein the controller reduces the number of eventsduring a sleep cycle and then increases the number of events duringnormal operating mode.
 28. A method of operation of a change detectionsensor, the method comprising: detecting ON events associated with lightreceived by pixels in a pixel array; assessing the events from the pixelarray; and changing how the pixels detect the events based on theassessment of the events from the pixel array.
 29. A method as claimedin claim 28, wherein assessing the events comprises counting the events.30. A method as claimed in claim 28, wherein assessing the eventscomprises estimating the events.
 31. A method as claimed in claim 28,further comprising setting thresholds applied by event detectors for thepixels in response to the assessment of the events.
 32. A method asclaimed in claim 28, further comprising setting thresholds for the ONevents and OFF events separately based on ON events and OFF events fromthe pixel array.
 33. A change detection sensor, comprising: a pixelarray comprising pixels that detect light, wherein the pixels comprisephotosensors and photoreceptor circuits that output photosensor signalsbased on the response of the photosensors; event detectors for detectingevents associated with light received by the pixels by comparing thephotosensor signals and a reference voltage; an event rate detector forassessing the events; and a controller for changing how the pixelsdetect the events based on the event rate detector to change the numberof events for different modes of the sensor.
 34. A sensor as claimed inclaim 33, wherein the modes include a sleep mode and a normal operatingmode.
 35. A sensor as claimed in claim 33, wherein a number of events isreduced for the sleep mode and increased for the normal operating mode.